(1) Field of the Invention
This invention relates to improvements in the techniques for detection of the velocity of movement of an optical head or the velocity of rotation of a spindle motor in an optical disk apparatus or a like apparatus.
(2) Description of the Prior Art
As optical disk apparatus, there are CD players (CD-ROM drives) and video disk players as well as optomagnetic files. In those optical disk apparatus which are positioned as peripheral equipments to computers, high speed data retrieval is required. To this end, conventionally the velocity of movement of an optical head is detected to effect velocity control of the optical head to realize a high speed moving operation of the optical head thereby.
In the velocity control, the number of tracking error signals produced for each unit time is counted to calculate velocity data. The calculation method is described below with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram showing an example of construction of a conventional velocity detection apparatus, and FIG. 2 is a timing chart illustrating operation of the velocity detection apparatus.
Referring to FIG. 1, a signal wherein a pulse is produced each time an optical head moves a fixed distance as seen in FIG. 2, for example, a tracking error signal, is used as position detection signal 43 to be inputted to binary counter 44. This position detection signal 43 presents the form of a pulse of a short period when the velocity of movement of the optical head is high, but presents the form of a pulse of a long period when the velocity of movement of the optical head is low. Accordingly, the difference in density of pulses of position detection signal 32 corresponds to the velocity of the optical head.
In the velocity detection apparatus shown in FIG. 1, binary counter 44 is reset by sampling signal 46 of a fixed period and counts the number of pulses of position detection signal 43 for each fixed period. D-type flipflop 47 latches data counted by binary counter 44 at the rising edge of each pulse of sampling signal 46 to produce velocity data 47.
The conventional velocity detection apparatus described above, however, has the following problems.
In particular, in the conventional velocity detection apparatus, when the velocity of movement of the optical head is high, good velocity data are obtained since the number of counted pulses is great. However, as the velocity decreases, the accuracy in detection of velocity data decreases since the period of position detection signal 43 approaches the period of sampling signal 46. On the other hand, if the period of the sampling signal increases, the number of pulses of position detection signal 43 per one sample increases to enhance the detection capacity of velocity data. However, since the sampling period becomes long, the capacity of controlling the velocity of movement is degraded.